Recent reports suggest that one way to inhibit vascular dysfunction is to change the apolipoprotein character of plasma lipoproteins. Treating cholesterol-fed LDLr(-/-) mice with an apolipoprotein A-1 mimetic, "4F," significantly reduces lesion formation (>75%) without significant changes in plasma cholesterol levels. Accordingly, the two major goals of this competitive renewal application are to determine the mechanisms by which low-density lipoprotein (LDL) impair and by which 4F preserves vasodilation during hypercholesterolemia. Recent reports from this laboratory demonstrate that native LDL and minimally modified (mm) LDL differentially regulate endothelial superoxide anion (02-) generation. Native LDL uncouples endothelial nitric oxide synthase (eNOS) activity while mm-LDL uncouples eNOS, activates xanthine oxidase and NAD(P)H oxidoreductase to increase endothelial 02- generation. Preliminary results indicate that many of the effects of LDL on endothelial 02- generation can be reversed by the apolipoprotein A-1 mimetic, 4F, which effectively turns LDL into a non-atherogenic lipoprotein. LDLr(-/-) mice fed high fat cholesterol western diets demonstrate impaired vasodilation to acetylcholine, where as vascular responses of microvessels from LDLr(-/-) mice treated with 4F are essential the same as control responses. These data suggest that 4F shifts the balance of nitric oxide (NO) and 02- back toward .NO to restore vasodilation. Cell biology studies have shown that although LDL and mm-LDL decrease hsp90 interactions with eNOS to promote uncoupled enzyme activity, adding 4F to LDL incubations restores hsp90 association with eNOS, increasing what appears to be coupled enzyme activity. On the basis that apo A-1 mimetics increase HDLlike function of plasma lipoproteins, our data suggest that HDL plays a critical role in decreasing endothelial 02"- generation, which is hypothesized to impair vasodilation during hypercholesterolemia. Investigations in to how 4F protects vascular function will provide new understanding of the mechanisms by which LDL impairs vasodilation and HDL protects vascular function.